Visualizing Categorical Distributions

Reading in the data

import altair as alt
import pandas as pd

movies_extended = pd.read_csv("../../data/movies-extended.csv")
movies_extended

	Title	US Gross	Worldwide Gross	US DVD Sales	...	Director	Rotten Tomatoes Rating	IMDB Rating	IMDB Votes
0	Boynton Beach Club	3127472.0	3127472.0	NaN	...	NaN	NaN	NaN	NaN
1	Broken Arrow	70645997.0	148345997.0	NaN	...	John Woo	55.0	5.8	33584.0
2	Brazil	9929135.0	9929135.0	NaN	...	Terry Gilliam	98.0	8.0	76635.0
...	...	...	...	...	...	...	...	...	...
1187	Zodiac	33080084.0	83080084.0	20983030.0	...	David Fincher	89.0	NaN	NaN
1188	The Legend of Zorro	45575336.0	141475336.0	NaN	...	Martin Campbell	26.0	5.7	21161.0
1189	The Mask of Zorro	93828745.0	233700000.0	NaN	...	Martin Campbell	82.0	6.7	4789.0

1190 rows × 16 columns

We’re continuing to work with the movies data set from the last slide deck, but here we will focus on the categorical dataframe columns.

Bar charts are effective for visualizing categorical “distributions” of a single column

alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()'),
    alt.Y('Major Genre', sort='x'))

We have already seen examples of visualizing categorical distributions when we used bar charts to plot the count of categories in previous modules.

Although these plots represent categorical distributions, it’s good to know that they are commonly referred to as just showing the counts of the categories, rather than their “distribution”.

Here we have made a bar chart to answer the question “Which is the most common genre among movies in this dataset?”.

We can see that the dataset consists of mostly comedies and dramas with very few concert/performance films.

Stacked bar charts can visualize counts for combinations of two categorical columns

alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()'),
    alt.Y('Major Genre', sort='x'),
    alt.Color('MPAA Rating'))

What if we wanted to ask a more complex question that involves visualizing the combinatorial counts of two categorical columns?

Our data contains the MPAA rating for each movie, which is a classification given by the Motion Picture Association indicating what age groups, a movie is suitable for:

- G – General Audiences
- PG – Parental Guidance Suggested
- PG-13 – Parents Strongly Cautioned
- R – Restricted

Do you think there are any differences in the proportions of ratings between movie genres?

For example, we might already have a hypothesis that there won’t be any horror movies that are appropriate for children.

To find out, we could colour the bars according to the MPAA rating as in this slide. However, one aspect of our chart is not very intuitive: the colour legend and the stacked bar segments are not sorted in the same order.

Reordering the bar segments aligns it with the order in the legend

alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()'),
    alt.Y('Major Genre', sort='x'),
    alt.Color('MPAA Rating'),
    alt.Order('MPAA Rating'))

Here we have reordered the bar segments using alt.Order, so that they are in the same order as the legend. We could have opted to reorder the legend instead, but it is more natural to start with the rating for children (G) and proceed towards increasingly more mature ratings.

In this chart we can see that most comedies are rated PG-13, most dramas are rated R, that action movies almost only have movies rated either R or PG-13

But this doesn’t really answer our question, which was if there are any differences in the proportions of MPAA ratings between movie genres?

While we can roughly see the relative proportions within a single bar, it is quite hard to compare the coloured segments between bars as proportions since the bars are of different lengths.

It is also impossible to see the proportions in the genres with fewer movies.

Rescaling the bar lengths facilitates comparing proportions between bars

alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()', stack='normalize', title='Proportion of movies'),
    alt.Y('Major Genre', sort='x'),
    alt.Color('MPAA Rating'),
    alt.Order('MPAA Rating'))

Here we have changed the strategy for how the segments are stacked together by setting stack='normalize', which means to normalize/rescale each bar to span the entire length of the x-axis and label it as a proportion.

The plot looks quite unordered, because ‘x’ is not a meaningful way to sort the bars when they are all the same length. Instead, we would like to sort them by the length of one of the coloured segments.

Sorting by the length of one of the coloured segments make the chart easier to read

sort_order = ['Adventure', 'Musical', 'Comedy', 'Romantic Comedy', 'Action',
              'Drama', 'Concert/Performance', 'Documentary', 'Western',
              'Thriller/Suspense', 'Horror', 'Black Comedy'] 
alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()', stack='normalize', title='Proportion of movies'),
    alt.Y('Major Genre', sort=sort_order),
    alt.Color('MPAA Rating'),
    alt.Order('MPAA Rating'))

That’s much better! Sorting the chart in an intuitive order has once again shown to be crucial for making our plot easier to interpret.

In a normalized stacked bar chart it makes sense to sort by either the first or the last coloured segment. Since not all genres have movies rated 'G', we chose to set the order based on 'R' instead.

If you are more interested in one genre than others, you could also choose to sort by that genre.

Now we can directly compare the length of individual bar segments between the genres. We see that there certainly are differences in the proportions of MPAA ratings between movies in different genres.

For the R rating, it is immediately obvious exactly how big these differences are, and we can see that all black comedies are rated R, while almost no adventure movies receive this rating.

It is a little bit harder to compare the other rating because the coloured segments do not share the same baseline. We can still see that concerts and performance movies has the highest proportions of movies that are kids-friendly at 50%, while musicals, documentaries, and action movies have around 20% kids-friendly movies.

(We specified the sort order manually in this slide, because it is rather advanced to extract this order using pandas, but we include an example in the transcript if you are interested in how this could be done:)

sort_order = (
    movies_extended
    .groupby('Major Genre')['MPAA Rating']
    .value_counts(normalize=True)
    .xs('R', level='MPAA Rating')
    .sort_values()
    .index
    .to_list())

Normalize stacked bar charts are effective at visualizing just a few categories

sort_order = ['Concert/Performance', 'Musical', 'Documentary', 'Adventure', 
              'Comedy', 'Romantic Comedy', 'Drama',  'Action']
alt.Chart(movies_extended[movies_extended['MPAA Rating'].isin(['G', 'PG'])]).mark_bar().encode(
    alt.X('count()', stack='normalize', title='Proportion of movies'),
    alt.Y('Major Genre', sort=sort_order),
    alt.Color('MPAA Rating'),
    alt.Order('MPAA Rating'))

As with all charts, there are shortcomings to normalized stacked bar charts as well. We saw in the last slide that it was a bit harder to compare the coloured segments that did not share a baseline versus the segments at the ends where the baseline is the same.

This means that normalized stacked bar charts are ideal when there are only two categories, since both segments will be easy to compare between categories.

We can see an example in this slide where we filtered the dataset to contain only movies rated either 'G' or 'PG'. It is easy to make a comparison for both the blue and the orange segments.

Stacked bar charts also work fine for 3-4 categories, but beyond that, they are usually ineffective. Even for 3-4 categories, it is often preferred to show the bars side-by-side instead of stacked.

Showing bars side by side makes it easier to compare their exact heights within a category

(alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()', title=''),
    alt.Y('MPAA Rating', title=''),
    alt.Color('MPAA Rating', legend=None))
 .properties(width=100, height=45)
 .facet('Major Genre', columns=4)
 .resolve_scale(x='independent'))

It is not (yet) possible to pass a different value to stack, which would put the bars next to each other. Instead, we could use faceting as in this slide.

Here, we have also removed some of the axis titles to make the figure more compact and less crowded with text. You will learn more about customizing elements such as titles in the next module.

By resolving the x-scale to be independent between the plots, the bar height is not relative to the max in each facet. This makes them easier to compare as proportions both within and between facets while still retaining an indication of the count on the x-axis.

If we wanted to compare the absolute counts between facets, we would leave the axis at its default value: “shared”.

This type of visualization is effective when we want to accurately compare the heights of bars within a genre. For example, to answer the question: “Which are the most common MPAA ratings for each genre?”.

Switching the faceting and y column targets the plot towards a slightly different question

(alt.Chart(movies_extended).mark_bar().encode(
    alt.X('count()', title=''),
    alt.Y('Major Genre', title='', sort='x'),
    alt.Color('MPAA Rating', legend=None))
 .properties(width=100, height=150)
 .facet('MPAA Rating')
 .resolve_scale(x='independent'))

If we instead wanted to answer the question: “Which are the most common genres for each MPAA Rating?”, we would switch the faceting and y columns.

Here, we kept the colour column as the MPAA Rating so that it is consistent with the previous figures, and because with this many categories, it would look quite messy if we coloured by the Major Genre instead.

We can see that the most common G-rated movies are adventure films and that most PG-13 rated movies are Comedies, Dramas and action movies. It is important to remember that genres have more movies in total, so, we would expect them to show up highly in all the facets. Whether this is desired or not depends on the question we are asking, but it does make sense here.

Heatmaps are effective for visualizing counts of two-dimensional categorical data

alt.Chart(movies_extended).mark_rect().encode(
    alt.Color('count()'),
    alt.X('MPAA Rating'),
    alt.Y('Major Genre', sort='color'))

If we want to get an overview of the information from both the faceted plots in the previous two slides, we could create a heatmap. In this heatmap the colour represents the combinatorial counts of two categorical columns, such as how many movies are both rated G and in the comedy genre.

Comparing squares vertically in this heatmap is similar to the first faceted plot we made and comparing them horizontally is similar to the faceted visualization in the last slide.

In other words, if we want to compare which genre is most common for a certain rating, we compare the colours column-wise in the heatmap. If we instead are interested in the most common rating assigned to a movie we compare the columns row-wise. We can quickly see that most dramas are rated PG-13 or R and most horror movies are rated R.

Sorting on colour/count puts the genres with many observations close together, similar to how we sorted on 'x' and 'y' in previous modules. We could sort the x-axis also, but since it has a natural order to it, we have decided to keep it as is here.

This visualization is effective for quickly communicating the main takeaways from the two questions and giving us an overview of the data, but it is harder to tell that exact count for each colour so, if that is of great importance a bar chart is more suitable.

Using both the colour and marker size to indicate the count creates a more effective visualization

alt.Chart(movies_extended).mark_circle().encode(
    alt.X('MPAA Rating'),
    alt.Y('Major Genre', sort='color'),
    alt.Color('count()'),
    alt.Size('count()'))

One potential concern with heatmaps is that they rely solely on colour to communicate the value of interest.

We cannot perceive small variations in colour as accurately as we can for other visual channels, such as the position or size.

Colour can also be problematic for people with colour vision deficiencies, which is almost 10% of the population, which we will talk more about in a later module.

To ameliorate these issues, we can use the same marks as when creating scatter plots, such as mark_circle or mark_square, which allows us to change the size of each mark in addition to its colour.

This visualization is highly effective in answering both of the questions we posed initially, and if we wanted to, we could now facet by a third categorical column such as the movie distributor, to interrogate three categorical columns simultaneously.

Let’s apply what we learned!